1. Field of the Invention
This invention relates to an aircraft leading edge and more particularly to an air intake of an aircraft nacelle that incorporates a coating for acoustic treatment as well as a system that makes it possible to prevent the formation and/or the accumulation of ice and/or frost.
2. Description of the Related Art
An aircraft propulsion unit comprises a nacelle in which a power plant that drives a fan mounted on its shaft is arranged essentially concentrically.
The nacelle comprises an inside wall that delimits a pipe with an air intake toward the front, a first portion of the entering air flow, called primary flow, passing through the power plant to participate in the combustion, the second portion of the air flow, called secondary flow, being driven by the fan and flowing into an annular pipe that is delimited by the inside wall of the nacelle and the outside wall of the power plant.
The noise emitted by the propulsion unit consists, on the one hand, of jet noise, produced on the outside of the pipes as a result of the mixing of various air flows and exhaust gases, and, on the other hand, of noise generated by the inside parts, called internal noise, produced by the fan, the compressors, the turbines and the combustion that propagates inside the pipes.
To limit the impact of noise pollution close to the airports, the international standards are increasingly restrictive as far as sound emissions are concerned.
Techniques have been developed to reduce the internal noise, in particular by using, at the walls of the pipes, coatings whose purpose is to absorb a portion of the sound energy, in particular by using the principle of Helmholtz resonators. In a known way, this acoustic coating, also called an acoustic panel, comprises—from the outside to the inside—an acoustically resistive porous layer, an alveolar structure, and a reflective layer.
Layer is defined as one or more layers that may or may not be of the same type.
The acoustically resistive porous layer is a porous structure that plays a dissipative role, partially transforming the acoustic energy of the sound wave that passes through it into heat. It comprises so-called open zones that are able to allow acoustic waves to pass and other so-called closed or full zones that do not allow the sound waves to pass but are designed to ensure the mechanical resistance of said layer. This acoustically resistive layer is characterized in particular by an open surface ratio that varies essentially based on the engine and the components that constitute said layer.
For the moment, because of various constraints, for example shaping or compatibility with other equipment, coatings are provided in particular at the inside wall of the nacelle on a limited zone that is distant from the air intake and the air discharge.
To increase the effectiveness of the acoustic treatment, one approach consists in increasing the surface areas that are covered by the acoustic coating. However, at the air intake or on the lip of the nacelle, the installation of an acoustic coating is not possible for the moment in particular because said coating is not compatible with the systems that make it possible to prevent the formation and/or the accumulation of ice and/or frost that are necessary in these zones.
These systems are divided into two large families, the first called defrosting systems that make it possible to limit the formation of ice and/or frost, the second called de-icing systems that limit the accumulation of ice and/or frost and act on both the ice and/or frost formed. Hereinafter, a frost treatment system is defined as a defrosting system or a de-icing system.
For the defrosting treatment, one approach consists in treating the aircraft on the ground by using a gas or a liquid that is deposited on the surfaces to be treated. Even if these treatments are effective, in particular at the time of take-off, they have a limited duration. It is necessary, however, that frost treatment systems be put on board the aircraft because the frost (or the ice) can form at the aerodynamic surface of the aircraft and more particularly at the leading edges of the wing, the nacelle, the stabilizer, etc., when the aircraft passes through certain meteorological conditions.
A first frost treatment system consists in using electric resistors that are made of a conductive material that is covered by an insulator to heat the surface to be treated by the Joule effect. This type of system is not satisfactory because it is relatively fragile and susceptible to being damaged by bird strikes, hail or accidents during maintenance. In the damaged zones, the frost treatment system can no longer function, making possible the formation and the accumulation of ice or frost. Furthermore, this type of frost treatment system has a relatively high electrical consumption so that it may be necessary for certain aircraft configurations to provide an additional power supply. Finally, it is not compatible with the coatings for the acoustic treatment because its presence on the surface generally changes the performance levels of the acoustic treatment.
Another frost treatment system consists in using hot air that is taken from the engine and fed back at the inside walls of the leading edges. This high-quality system is not very compatible with the acoustic treatment coating to the extent that the latter is relatively thick and consists of cavities that contain air and that act as an insulating material. Furthermore, the use of a hot-air system may prove problematic under certain flight conditions—in particular at the moment of deceleration before landing—that are generally the most likely times for frost to appear, to the extent that the production of hot air by the engine is reduced during these phases.
Finally, this type of frost treatment system also has the drawback of generating significant temperatures and pressure variations.